Recently, the method for fabricating HARMs (High Aspect Ratio Microstructures) by utilizing SCS (Single Crystal Silicon) exceeds the shape limitation of the traditional bulk micromachining process and thereby can diversify the fabrication of a thick structure. Compared with a thin film structure in the surface micromachining process, the thick structure provides several advantages including the smaller dynamic deformation, the higher driving frequency, the tiny influence of the plane disturbance, and the large range of the rigidity of the structure. Furthermore, if the thick structure can be actuated and detected electrostatically, the static electricity and the capacitance of the structure will become larger owing to the increase of the thickness, and thus the efficiency of detection and actuation will be improved substantially. In addition, the single crystal silicon employed to form the main structure is a widely used material in MEMS (Micro-Electro-Mechanical System) even if single crystal silicon is brittle. However, the brittleness-destroying probability could be reduced by controlling the defects of the SCS because the sizes of MEMS elements are usually small, thereby increasing the flexibility thereof. Besides, the mechanical strength of the SCS is greater than that of most metals and alloys. Thus the SCS do not have the thin film stress problem. Furthermore, since the thermal expansion coefficient of the SCS is smaller than that of the metals, the signal shift phenomenon induced by temperature variation will become smaller. Additionally, in contrast, the Young's modulus and the Poisson's ratio in all orientations on the (111) silicon wafer surface still have the same superiority and can reduce the error caused by the different mechanical characteristics of different crystal lattices. In view of the aforesaid, the HARMs bulk micromachining process with the (111) SCS can actually satisfy numerous design demands of the MEMS elements.
The (111) plane is the etch-stop plane of silicon wet etching in an aqueous alkaline etchant. (111) SCS is directed to a wafer whose (111) lattice planes all are vertical to the normal. The general HARMs bulk micromachining process of the (111) SCS utilizes the specific (111)-oriented silicon to cooperate the anisotropic side etching and thereby to fabricate the main structure which can be released as an actuated structure via etching having a large-area mass with paralleled top and bottom surfaces. Please refer to FIGS. 1A˜1F. FIGS. 1A˜1F show the flowchart of a typical fabrication method of HARMs actuator for employing the (111) wafer. First, a silicon substrate 11 is provided to form the electrode 12 of the actuator by selectively diffusing the dopants, and then a masking layer 13 which usually can be silicon oxide having high etching selectivity is grown thereon (as shown in FIG. 1A). Second, the masking layer 13 is patterned and then been deep etched to define the depth of the structure layers (as shown in FIG. 1B). Third, for protecting the sidewall, the silicon oxide film 14 having great sidewall attachment is grown thereon (as shown in FIG. 1C). Fourth, the RIE (Reactive Ion Etching) is executed for removing the silicon oxide thereunder and to be beneficial to the next silicon deep etching (as shown in FIG. 1D). Fifth, the deep etching is executed to define the depth of the sacrificed layer (as shown in FIG. 1E). Sixth, the anisotropic etching and the convex-corner undercut of the (111) wafer are employed to release the main structure 15 as an actuated structure for accomplishing the whole processes (as shown in FIG. 1F). However, although the (111) wafer has the specific characteristic of side etching, if the etching time is out of control, it will cause the post area 16 structural unstable. Moreover, the post area 16 will be undercut and more seriously to affect the stability of the structure even the electricity. Among the practical structures, the undercut is avoided in many post areas and these post area can be closely connected to the substrate, such as the boundary anchor of the releasing structure, the bonding pad of the electrode, and the wafer bonding area. For avoiding these post areas being convex-corner undercut, the most direct method is to make the width of the post areas greatly larger than the width of the released areas. But this will cause the occupied area too big and limit the design. For reducing the post areas, except helping the design of the structural shape and orientation, the traditional corner compensation, which extends the corners partially, also can increase the undercutting time of the post areas. But the design of the plane is still inhibited, because the occupied post areas are large. Consequently, the smaller post area still can not be fabricated under this condition.
In addition, although the (111) silicon wafer can fabricate many structures which can not be accomplished by the surface micromachining process, there still has a problem of insufficient conductive depth. Because the depth of silicon diffusion is always limited to be less than 10 micrometers, some deep portions of the thick structures can not own the conductivity, and thus the characteristics of actuation and detection are also influenced. Although this problem can be solved by attaching a conductive/dielectric layer thereon, some sequential fabrication problems will still be shown, e.g., the difficulty of wafer connecting, the uneasy attachment of the metal sidewall, and the machine limitation. All these will lose the advantages of the single crystal silicon.
Because of the technical defects described above, the applicant keeps on carving unflaggingly to develop “corner compensation method for fabricating MEMS and structure thereof” through wholehearted experience and research.